Support `oneOf` semantic in schema

To support `oneOf` semantic in the schema — an example use case is where one has a Rust enum type that contains structs, e.g.
```rust
pub enum Enum {
Enum1(Struct1),
Enum2(Struct2),
}
```
which can easily be defined in the schema as
```yaml
Enum:
oneOf:
$ref: "#/components/schemas/Struct1"
$ref: "#/components/schemas/Struct2"
```